The present invention relates to a vapour compression system operating at both subcritical and supercritical high-side pressures.
In conventional vapour compression systems, the high-side pressure is determined by the condensing temperature, via the saturation pressure characteristics of the refrigerant. The high side pressure in such systems is always well below the critical pressure.
In vapour compression systems operating with supercritical high-side pressure, i.e. in a trans-critical cycle, the operating pressure depends on several factors such as momentary refrigerant charge in the high side, component volumes and temperature of heat rejection.
A simple vapour compression system with an expansion device of conventional design, e.g. of the thermostatic type, would also be able to provide trans-critical cycle operation when the heat rejection temperature is above the critical temperature of the refrigerant. Such a system could give a simple and low-cost embodiment for a trans-critical vapour compression cycle using environmentally benign refrigerants such as CO.sub.2. This simple circuit does not include any mechanisms for high-side pressure modulation, and the pressure will therefore be determined by the operating conditions and the system design.
A serious drawback in trans-critical operation of a system that is designed in accordance with common practice from conventional subcritical units is that, most likely, a relatively low refrigerating capacity and a poor efficiency will be obtained, due to far from optimum high side pressures during operation. This will result in a considerable reduction in capacity as supercritical conditions are established in the high side of the circuit. The loss in refrigerating capacity may be compensated for by increased compressor volume, but then at the cost of significantly higher power consumption and higher investments.
Another major disadvantage in trans-critical operation of a conventionally designed system is that leakage of refrigerant will immediately affect the high side pressure, due to the reduction in high-side charge. At supercritical high side conditions, the pressure is determined by the relation between instant refrigerant charge and component volumes, similar to the conditions in a gas-charged pressure vessel.
Still another disadvantage is that excessive pressures can easily build up in a fully charged non-operating system subjected to high ambient temperatures. The latter effect can cause damage, or can be taken into account in the design, but then at the cost of heavy, voluminous and expensive components and tubes.
It is therefore a major object of the present invention to provide a simple, efficient and reliable vapour compression system avoiding these and other shortcomings.